Method and apparatus for impedance signal localizations from implanted devices

ABSTRACT

A patient monitoring system including an implantable medical device for monitoring a plurality of physiological factors contributing to physiological conditions of a patient&#39;s heart by measuring a first impedance affected by the plurality of physiological factors, across one of a plurality of vectors, and a second impedance affected by the plurality of physiological factors, across a second one of the plurality of vectors subsequent to determining the first impedance. A change in impedance is determined based upon the first impedance and the second impedance measurements. Using an equation ΔZ VX =α AVX *Q A +α BVX *Q B , where Q A  is a fractional resistivity change of a first contributing physiological impedance factor, Q B  is a fractional resistivity change of a second physiological impedance factor, α AVX  is an impedance sensitivity factor for physiological impedance factor Q A , and α BVX  is an impedance sensitivity factor for physiological impedance factor Q B , the value of one of the contributing physiological impedance factors is determined. The contributing physiological impedance factors may include lung resistivity, blood resistivity, heart muscle resistivity, skeletal muscle resistivity, heart volume and lung volume.

FIELD OF THE INVENTION

The present invention relates generally to implantable medical devices(IMDs), and more particularly, the present invention relates to anapparatus and method for identifying cardiac insult using comparisons ofmultiple impedance vectors to differentiate between the physiologicalfactors that contribute to cardiac insult.

BACKGROUND OF THE INVENTION

The impedance measuring vectors or paths provided by some modernpacemakers and implantable cardio defibrillators are quite extensive.Many pacemakers currently measure impedance to measure minuteventilation as a physiological indicator of activity. The minuteventilation value obtained in this way can be used to set the pacingrate in a physiological adaptive pacemaker. The impedance changes overtime over a particular vector can have many contributing factors, somemajor and some minor, so that multiple factors contribute to impedancesignals measured by the device. A nonexclusive list of such contributingfactors in which changes in the factors over time can cause changes inthe measured impedance over time across a vector include, for example,changes in lung resistivity, changes in blood resistivity, changes inheart muscle resistivity, changes in skeletal muscle resistivity,changes in heart volume, and changes in lung volume. Measuring changesin impedance or resistivity in a certain contributing factor can beproblematic, since such changes tend to be relatively accuratelydetectable across one vector while being less susceptible to accuratedetection across another vector. Some vectors are highly sensitive orsusceptible to changes in certain of the contributing factors, whilebeing less sensitive or susceptible to impedance changes in othercontributing factors.

What is needed is a method and apparatus that more accuratelydifferentiates between the multiple sources of and/or physiologicalfactors that contribute to changes in impedance measures over time.

SUMMARY OF THE INVENTION

The present invention is directed to apparatus for monitoring aplurality of physiological factors contributing to physiologicalconditions of a patient. The patient monitoring system includes animplantable medical device having a housing and a connector blockconfigured to couple to a cardiac lead system having a plurality ofelectrodes. Selected ones of the electrodes of a cardiac lead system areused to establish an impedance vector in tissue proximate a patient'sheart. The impedance of tissue proximate a patient's heart is measuredbased upon the impedance vector established by the selected electrodes.A determination is made as to a quantifying value for a contributingphysiological impedance factor among a plurality of physiologicalimpedance factors associated with a physiological condition of apatient's heart. The plurality of physiological impedance factorsincludes lung resistivity, blood resistivity, heart muscle resistivity,skeletal muscle resistivity, heart volume and lung volume.

According to a preferred embodiment, a microprocessor is used to makethe determination as to a quantifying value for a contributingphysiological impedance factor and operates to

-   -   (1) cause a means for measuring impedance of tissue proximate a        patient's heart based upon an impedance vector formed between        electrodes of a cardiac lead system to make first and second        impedance measurements spaced apart in time along a first        impedance vector and to make first and second impedance        measurements spaced apart in time along a second impedance        vector;    -   (2) calculate a value for a change in measured tissue impedance        over time along each of the first and second impedance vectors        as ΔZ_(V1) and ΔZ_(V2), respectively;    -   (3) insert each of the calculated values ΔZ_(V1) and ΔZ_(V2)        into an equation        ΔZ _(VX)=α_(AVX) *Q _(A)+α_(BVX) *Q _(B)    -   where Q_(A) is a first fractional resistivity change of a        physiological impedance factor,        -   Q_(B) is a second fractional resistivity change of a            physiological impedance factor,        -   α_(AVX) is an impedance sensitivity factor for physiologcal            impedance factor Q_(A), and        -   α_(BVX) is an impedance sensitivity factor for physiological            impedance factor Q_(B);    -   (4) subtract ΔZ_(V2) from ΔZ_(V1) to form the equation        ΔZ _(V1) −ΔZ _(V2)=(α_(AV1)−α_(AV2))*Q _(A)+(α_(BV1)−α_(BV2))*Q        _(B);and    -   (5) solve for a quantifying value for one of the physiological        impedance factors Q_(A) and Q_(B) using the equation.        Further in accordance with the preferred embodiment, Q_(A) and        Q_(B) are selected from a group of physiological impedance        factors consisting of lung resistivity, blood resistivity, heart        muscle resistivity, skeletal muscle resistivity, heart volume        and lung volume.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features of the present invention will be readilyappreciated as the same becomes better understood by reference to thefollowing detailed description when considered in connection with theaccompanying drawings, in which like reference numerals designate likeparts throughout the figures thereof and wherein:

FIG. 1 is a schematic diagram of impedance vectors crossing twophysiological impedance change factors;

FIG. 2 is a schematic diagram of an exemplary implanted medical devicesystem for measuring impedance changes across and/or near a heartaccording to the present invention;

FIG. 3 is a functional schematic diagram of an implantable medicaldevice in which the present invention may be practiced;

FIG. 4 is a table of sensitivity or susceptibility coefficients ofseveral vectors to changes in impedance in several physiological factorimpedance contributors; and

FIG. 5 is a flowchart illustrating a method for isolating impedancechanges over time to physiological factors according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed description should be read with reference to thedrawings, in which like elements in different drawings are numberedidentically. The drawings, which are not necessarily to scale, depictselected embodiments and are not intended to limit the scope of theinvention. Several forms of invention have been shown and described, andother forms will now be apparent to those skilled in art. It will beunderstood that embodiments shown in drawings and described above aremerely for illustrative purposes, and are not intended to limit scope ofthe invention as defined in the claims that follow.

FIG. 1 is a schematic diagram of impedance vectors crossingphysiological impedance change factors. As illustrated in FIG. 1, anabstract diagram 100 illustrating a simplified example of the presentinvention includes one physiological factor contributing to changes inimpedance over time as sensed across various vectors, Factor A, andanother physiological factor contributing to changes in impedance overtime as sensed across various vectors, Factor B. Impedance changecontributing Factor A is represented at 102 and impedance changecontributing Factor B is indicated at 104. A region of overlap 106 isformed that includes contributing Factor A and contributing Factor B.Three vectors, Vector 1 at 108, Vector 2 at 112, and Vector 3 at 110,are also illustrated. Vector 1 conceptually passes through a largeportion of Factor A, while being little influenced by Factor B. Vector 2passes through a large portion of Factor B, being little influenced byFactor A. Vector 3 passes through portions of both Factor A and Factor Band is thus influenced somewhat by both Factor A and Factor B. As aresult, the sensitivity or susceptibility of Vector 1 to Factor A ishigh, and the sensitivity or susceptibility of Vector 1 to Factor B islow. The sensitivity of Vector 2 to Factor A is low and the sensitivityof Vector 2 to Factor B is high. The sensitivity of Vector 3 to Factor Ais medium, as is the sensitivity of Vector 3 to Factor B.

Generally, the change in impedance over time across a Vector X in thesimplified system of FIG. 1 is given in Equation 1 below.ΔZ _(VX)=α_(VXA) *Q _(A)+α_(VXB) *Q _(B)  (1)

The term α_(VXA) in Equation 1 is the sensitivity to impedance changesover time across Vector X caused by resistivity changes over time inFactor A. Similarly, α_(VXB) is used to indicate the changes over timeacross Vector X caused by resistivity changes over time in Factor B.Q_(A) indicates the relative change in resistivity over time in Factor Aand Q_(B) indicates the relative change in resistivity over time inFactor B.

Equation 2 below gives the changes in impedance over time across anothervector, Vector Y.ΔZ _(VY)=α_(VYA) *Q _(A)+α_(VYB) *Q _(B)  (2)

Equation 2 states that the changes in impedance over time across VectorY are equal to the sensitivity to changes over time across Vector Ycaused by resistivity changes over time in Factor A times the fractionalresistivity changes over time in Factor A plus the sensitivity tochanges over time across Vector Y caused by resistivity changes inFactor B over time times the fractional change in resistivity over timein Factor B.Q _(A)=Δρ_(A)/ρ_(A)=(ρ_(AT2)−ρ_(AT1))/ρ_(AT1)  (3)

Equation 3 indicates that the fractional change (relative change orpercentage change) in resistivity of Factor A is equal to the change inthe resistivity of Factor A relative to the resistivity of Factor A.This may also be stated as indicated in Equation 3, as being the changein resistivity from Time 1 to Time 2 divided by the resistivity at Time1.

Taken together, Equations 1 and 2 provide a system of equations that canbe solved. These equations can be easily solved, even in the presence ofadditional factors, if the sensitivity coefficients, the α values, arenot randomly occurring but have advantageous patterns. In particular,where there are multiple vectors available to select from, it will beadvantageous to select Vectors X and Y such that the sensitivity valuesα_(VX) and α_(VY) differ only for one factor. To find or evaluate Q_(A)in Equations 1 and 2, it is advantageous to find two vectors, X and Y,such that α_(VXA) is substantially different than α_(VYA) and such thatα_(VXB) is substantially equal to α_(VYB). It may be more generallystated, that in order to solve for relative changes in Factor A overtime, the sensitivity to changes in Factor A across Vectors X and Yshould differ from each other, while the sensitivities across Vectors Xand Y should be substantially equal for any remaining factors in whichthe change in impedance over time is not known and for which thecontribution is significant.

Referring again to FIG. 1, in order to evaluate Q_(A) (the relativechange in resistivity in Factor A), Vectors 1 and 3 may be selected.Substituting Vectors 1 and 3 into Equations 1 and 2 results in Equations4 and 5 below.ΔZ _(V1)=α_(V1A) *Q _(A)+α_(V1B) *Q _(B)  (4)ΔZ _(V3)=α_(V3A) *Q _(A)+α_(V3B) *Q _(B)  (5)

Equation 6 below results from subtracting equation 5 from equation 4.ΔZ _(V1) −ΔZ _(V3)=(α_(V1A)−α_(V3A))*Q _(A)+(α_(V1B)−α_(V3B))*Q_(B)  (6)

Solving for Q_(A) we arrive at Equation 7 below.Q _(A)=(ΔZ _(V1) −ΔZ _(V3))/(α_(V1A)−α_(V3A))  (7)

As previously discussed, α_(V1B) and α_(V3B) are substantially equal toeach other, and therefore are either zero or a very small value and maythus be ignored. In systems where the number of equations equals thenumber of unknowns, it is possible to use standard matrix algebra tosolve for Q_(A) and Q_(B). As is discussed later, there may not alwaysbe a number of equations equal to the number of unknowns, but the factorchanges in resistivity may still be evaluated due to similarities anddifferences in values of the susceptibility coefficients. Equation 7thus indicates that given the susceptibility values, and given themeasured impedance changes over time for Vector 1 and Vector 3, theresistivity changes over time in Factor A can be evaluated. As will bediscussed later, the resistivity changes over time for a single factormay be highly physiologically significant, and can serve as an indicatorof the progress of specific medical conditions.

The system of equations above can be further extended to include otherfactors.ΔZ _(VX)=α_(VXA) *Q _(A)+α_(VXB) *Q _(B)+α_(VXC) *Q _(C)  (8)ΔZ _(VY)=α_(VYA) *Q _(A)+α_(VYB) *Q _(B)+α_(VYC) *Q _(C)  (9)ΔZ _(VZ)=α_(VZA) *Q _(A)+α_(VZB) *Q _(B)+α_(VZC) *Q _(C)  (10)

Equations 8, 9 and 10 above include a new factor, Factor C. A newvector, Vector Z, is also included. It may be noted that while equation8 is shown for completeness, it is not needed to solve for Q_(B) ifQ_(A) is known and α_(VXC) is substantially equal to α_(VYC). To solvefor Q_(B), since Q_(A) is known, we can select vectors such that thesensitivity varies between the selected vectors only for Factor B, andnot Factor C, with Factor A being taken care of already by the knownvalue of Q_(A). Selecting Vectors Y and Z leads to Equation 11 below.ΔZ _(VY) −ΔZ _(VZ)=(α_(VYA)−α_(VZA))*Q _(A)+(α_(VYB)−α_(VZB))*Q_(B)+(α_(VYC)−α_(VZC))*Q _(C)  (11)

Solving for Q_(B) leads to Equation 12 below.Q _(B)=((ΔZ _(VY) −Δ _(ZVX))−(α_(VYC)−α_(VZC))*Q_(C)−(α_(VYA)−α_(VZA))*Q _(A))/(α_(VYB)−α_(VZB))  (12)

As α_(VYC) and α_(VZC) were selected to be substantially equal to eachother, the difference of these two terms is very small relative toα_(VYB)−α_(VZB) or zero and drops out of the above equation. Therefore,Q_(B) is solved. It should be noted that Factor C in the above equationcould be a grouped or lumped factor. This can prove useful where thegrouped or lumped factor is an indicator as a grouped or lumped factorof a significant medical condition.

FIG. 2 is a schematic diagram of an exemplary implanted medical devicesystem for measuring impedance changes across and/or near a heartaccording to the present invention. As illustrated in FIG. 2, animplantable medical device system 10 includes an implantable cardiacdefibrillator (ICD) 12 having a housing or can 14 and a connector block16. IMD system 10 may be implemented using any of a number of medicaldevices or alternative device configurations, including, but not limitedto ICD 12. Other techniques or therapies responsive to electrocardiogram(EGM) signals or other patient diagnostic data, such as therapies thatadminister drugs in response to atrial arrhythmia, also may implementvarious embodiments of the invention.

IMD system 10 includes a ventricular lead, which includes an elongatedinsulated lead body 24, carrying three concentric coiled conductorsseparated from one another by tubular insulative sheaths. The distal endof the ventricular lead is deployed in right ventricle 38. Locatedadjacent the distal end of the ventricular lead are a ring electrode 40,an extendable helix electrode 44, mounted retractably within aninsulative electrode head 42, and an elongated (approximately 5 cm)defibrillation coil electrode 36. Defibrillation electrode 36 may befabricated from many materials, such as platinum or platinum alloy. Eachof the electrodes is coupled to one of the coiled conductors within leadbody 24.

Electrodes 40 and 44 are employed for cardiac pacing and for sensingventricular depolarizations. Accordingly, electrodes 40 and 44 serve assensors for a ventricular electrocardiogram (V-EGM). At the proximal endof the ventricular lead is a bifurcated connector 20 that carries threeelectrical connectors, each coupled to one of the coiled conductors.

The right ventricular (RV) lead includes an elongated insulated leadbody 22, carrying three concentric coiled conductors, separated from oneanother by tubular insulative sheaths, corresponding to the structure ofthe ventricular lead. The distal end of the RV lead is deployed in rightatrium 34. Located adjacent the distal end of the RV lead are a ringelectrode 32 and an extendable helix electrode 28, mounted retractablywithin an insulative electrode head 30. Each of the electrodes iscoupled to one of the coiled conductors within lead body 22. Electrodes28 and 32 are employed for atrial pacing and for sensing atrialdepolarizations. Accordingly, electrodes 28 and 32 serve as sensors foran atrial electrocardiogram (AEGM).

An elongated coil electrode 26 is provided proximal to electrode 32 andcoupled to the third conductor within lead body 22. Electrode 26 ispreferably at least 10 cm long and is configured to extend from the SVCtoward the tricuspid valve. At the proximal end of the lead is abifurcated connector 18 that carries three electrical connectors, eachcoupled to one of the coiled conductors.

Implantable ICD 12 is shown in combination with the leads, with leadconnector assemblies 18 and 20 inserted into connector block 16. Outwardfacing portion of housing or can 14 of ICD 12 may be left uninsulated sothat the uninsulated portion of the housing or can 14 optionally servesas a subcutaneous defibrillation electrode, used to defibrillate eitherthe atria or ventricles. In addition, a button electrode 158 may also beincluded along housing 14.

FIG. 3 is a functional schematic diagram of an implantable medicaldevice in which the present invention may be practiced. FIG. 3 should beconstrued as an illustrative example of one type of device in which theinvention may be embodied. The invention is not limited to theparticular type of device shown in FIG. 3, but may be practiced in awide variety of device implementations, such as a pacemaker or an ICD.In addition, the invention is not limited to the implementation shown inFIG. 3. For example, the invention may be practiced in a system thatincludes more or fewer features than are depicted in FIG. 3.

The device illustrated in FIG. 3 is provided with an electrode systemincluding electrodes. For clarity of analysis, the pacing/sensingelectrodes 100, 102, 104, and 106 are shown as logically separate frompacing/defibrillation electrodes 152, 154, 156 and 158. Electrodes 152,154, 156 and 158 correspond respectively to an atrial defibrillationelectrode, a ventricular defibrillation electrode, the uninsulatedportion of the housing of the implantable PCD and a button electrodepositioned along the housing. Electrodes 152, 154, 156 and 158 arecoupled to a high voltage output circuit 144. High voltage outputcircuit 144 includes high voltage switches controlled bycardioversion/defibrillation (CV/defib) control logic 142 via a controlbus 146. The switches within output circuit 144 control which electrodesare employed and which are coupled to the positive and negativeterminals of a capacitor bank including capacitors 159 and 160 duringdelivery of defibrillation pulses.

Electrodes 104 and 106 are located proximate a ventricle and are coupledto an R-wave sense amplifier 114. Operation of amplifier 114 iscontrolled by pacing circuitry 120 via control lines 116. Amplifier 114may perform other functions in addition to amplification, such asfiltering signals sensed by electrodes 104 and 106. Amplifier 114 mayalso include a comparator that compares the input signal to apreselected ventricular sense threshold. Amplifier 114 outputs a signalon an R-out line 118 whenever the signal sensed between electrodes 104and 106 exceeds the ventricular sense threshold.

Electrodes 100 and 102 are located on or in an atrium and are coupled toa P-wave sense amplifier 108. Operation of amplifier 108 is controlledby pacing circuitry 120 via control lines 110. Amplifier 108 may performother functions in addition to amplification, such as filtering signalssensed by electrodes 100 and 102. Amplifier 108 may include a comparatorthat compares the input signal to a preselected atrial sense threshold,which is usually different from the ventricular sense threshold.Amplifier 108 outputs a signal on a P-out line 112 whenever the signalsensed between electrodes 100 and 102 exceeds the atrial sensethreshold.

A switch matrix 134 selectively couples the available electrodes to awide band (2.5–150 Hz) amplifier 136 for use in signal analysis. Signalanalysis may be performed using analog circuitry, digital circuitry, ora combination of both.

A microprocessor 128 controls the selection of electrodes via adata/address bus 126. The selection of electrodes may be varied asdesired. Amplifier 136 provides signals from the selected electrodes toa multiplexer 138, which provides the signals to an analog-to-digital(A/D) converter 140 for conversion to multi-bit digital signals and to arandom access memory (RAM) 130 under control of a direct memory access(DMA) circuit 132 for storage.

The PCD illustrated in FIG. 3 also contains circuitry for providingcardiac pacing, cardioversion, and defibrillation therapies. Forexample, pacer timing/control circuitry 120 may include programmabledigital counters that control the basic time intervals associated withDDD, VVI, DVI, VDD, AAI, DDI, and other modes of single and dual chamberpacing. Pacer timing/control circuitry 120 may also control escapeintervals associated with anti-tachyarrhythmia pacing in both the atriumand the ventricle, employing any of a number of anti-tachyarrhythmiapacing therapies.

Intervals defined by pacing circuitry 120 include, but are not limitedto, atrial and ventricular pacing escape intervals, refractory periodsduring which sensed P-waves and R-waves are ineffective to restarttiming of the escape intervals, and pulse widths of the pacing pulses.Microprocessor 128 determines the durations of these intervals based onstored data in RAM 130 and communicates these durations to pacingcircuitry 120 via address/data bus 126. Microprocessor 128 alsodetermines the amplitude of pacing pulses and communicates thisinformation to pacing circuitry 120.

During pacing, pacing timing/control circuitry 120 resets its escapeinterval counters upon sensing P-waves and R-waves as indicated bysignals on lines 112 and 118. The escape interval counters are reset inaccordance with the selected mode of pacing on time-out triggergeneration of pacing pulses by pacer output circuits. These pacer outputcircuits include an atrial pacer output circuit 122 coupled toelectrodes 100 and 102, and a ventricular pacer output circuit 124coupled to electrodes 104 and 106. Pacing timing/control circuitry 120also resets the escape interval counters when the pacer output circuitsgenerate pacing pulses, thereby controlling the basic timing of cardiacpacing functions, including anti-tachyarrhythmia pacing. Microprocessor128 determines the durations of the intervals defined by the escapeinterval timers and communicates these durations using data/address bus126. The value of the count present in the escape interval counters whenreset by sensed R-waves and P-waves may be used to measure the durationsof R—R intervals, P-P intervals, P-R intervals, and R-P intervals. Thesemeasurements are stored in RAM 130 and used to detect tachyarrhythmias.

Microprocessor 128 typically operates as an interrupt-driven deviceunder control of a program stored in an associated read only memory(ROM, not shown) and is responsive to interrupts from pacertiming/control circuitry 120 corresponding to the occurrence of sensedP-waves and R-waves and to the generation of cardiac pacing pulses.Data/address bus 126 provides these interrupts. In response to theseinterrupts, microprocessor 128 performs any necessary mathematicalcalculations, and pacer timing/control circuitry 120 may update thevalues or intervals that it controls.

When an anti-tachyarrhythmia pacing regimen is indicated based on adetected atrial or ventricular tachyarrhythmia, appropriate timingintervals are loaded from microprocessor 128 into pacer timing/controlcircuitry 120. In the event that generation of a cardioversion ordefibrillation pulse is required, microprocessor 128 employs an escapeinterval counter to control timing of such cardioversion anddefibrillation pulses, as well as associated refractory periods.

In response to the detection of atrial, ventricular fibrillation ortachyarrhythmia requiring a cardioversion pulse, microprocessor 128activates cardioversion/defibrillation control circuitry 142, which useshigh voltage charging control lines 150 to cause a charging circuit 162to initiate charging of high voltage capacitors 158 and 160. A VCAP line148 monitors the voltage on high voltage capacitors 158 and 160 andcommunicates this information through multiplexer 138. When this voltagereaches a predetermined value set by microprocessor 128, A/D converter140 generates a control signal on Cap Full (CF) line 164 to terminatecharging. Thereafter, pacer timing/control circuitry 120 controls timingof the delivery of the defibrillation or cardioversion pulse. Followingdelivery of the fibrillation or tachyarrhythmia therapy, microprocessor128 returns the device to cardiac pacing and waits for a subsequentinterrupt due to pacing or the occurrence of a sensed atrial orventricular depolarization.

An output circuit 144 delivers the cardioversion or defibrillationpulses as directed by control circuitry 142 via control bus 146. Outputcircuit 144 determines whether a monophasic or biphasic pulse isdelivered, the polarity of the electrodes, and which electrodes areinvolved in delivery of the pulse. Output circuit 144 may include highvoltage switches that control whether electrodes are coupled togetherduring delivery of the pulse. Alternatively, electrodes intended to becoupled together during the pulse may simply be permanently coupled toone another, either inside or outside the device housing. Similarly,polarity may be preset in some implantable defibrillators.

An impedance measurement logical interface (LIMLI) 180 is provided andemployed when initiated by microprocessor 128 on address/data bus 126either automatically on a periodic basis or in response to a programmedcommand received through telemetry. According to the present invention,impedance is measured along selected vectors extending through thetissue of the body using various electrodes, as will be described belowin detail.

One embodiment of the invention utilizes a pacing device, havingfirmware adapted to stimulate tissue at sub-threshold levels and tosense various impedance values across various vectors using variouselectrodes coupled to the device. Presently available implanted cardiacdevices have impedance sensing capability that is used to measure minuteventilation and physiological activity. Circuitry and systems suitablefor stimulating cardiac tissue and measuring impedance across the tissueis described, for example, in U.S. Pat. No. 5,562,711 (Yerich et al.),herein incorporated by reference. Other impedance measuring circuitry isdisclosed in U.S. Pat. No. 6,070,100 (Bakels et al.), hereinincorporated by reference.

It is to be understood that the impedance measurements include “raw”measurements and “processed” measurements. Processed measurementsinclude “average” measurements formed of the averages of more than onemeasurement, “filtered” measurements formed of filtered impedancemeasurements, “derivative” impedance measurements formed of the first orhigher order derivatives of impedance measurements, “selected” impedancemeasurements formed of the highest or lowest impedance measurements froma set of impedance measurements, “gated” impedance measurements takenfrom peaks or troughs in or gated to respiratory or cardiac cycles, and“inverted” impedance measurements formed of inverted impedancemeasurements. The selected impedance measurements can be used to catchan impedance minimum or maximum from a time region including a smallnumber, for example, 1 to 10, of impedance measurements. In embodimentshaving more than one pair of sensing electrodes, two or more sensingelectrode impedance measurements can be added together to form an“augmented” impedance measurement. Similarly, one or more sensingelectrode measurement can be subtracted from one or more other sensingelectrode measurement to form a “subtracted” impedance measurement. Boththe augmented and subtracted impedance measurements can provide valuableinformation gathered from the similarities or differences encountered bythe stimulating current's path to the sensing electrodes. Unless notedotherwise, the impedance measurements used in all methods according tothe present invention can be any of the aforementioned raw and processedimpedance measurements and combinations thereof.

It also is to be understood that the system depicted here need not belimited to these lead positions, electrode sizes, and numbers ofelectrodes. Other embodiments of this system include multi-polarelectrodes (3 or more electrodes on a single lead), defibrillationcoils, and/or the pacemaker can and/or button electrodes on the can. Insome embodiments, the impedance measurement can be made between two ormore stimulating electrodes and two or more sensing electrodes. whichare not necessarily exclusive of each other. Specifically, some of thestimulating electrodes may also be sensing electrodes.

Various paths or vectors may be drawn between any combination ofelectrodes connected to implanted device can 14 or connected to leads 22and 24. One electrode may serve as an emitter while another electrodemay serve as a collector, with yet another pair of electrodes used tomeasure the electrical potential between those electrodes, to determinethe impedance across the paths or vectors. The emitter and collector canshare one or both electrodes with the electrode pair used to sense thevoltage, in bipolar and tripolar configurations, respectively. Inquadrapolar configurations, the emitter, collector, and measuringelectrode pair are distinct electrodes. The term “vector” and “path” maybe used interchangeably for the purposes of the present application.

For example, a first vector used to measure impedance changes, Vector 1,is formed by a stimulation path and a sense path between RV coil 36 andcan 14. Vector 1 is a bi-polar vector, utilizing RV coil 36 as theemitter and can 14 as the collector, and measuring voltage at RV coil 36and can 14. Impedance changes may also be measured across anothervector, Vector 2, formed by a stimulation path from RV ring electrode(Vr) 40 to can 14 and a sense path from RV coil 36 to can 14. Vector 2is a tri-polar vector, utilizing RV ring 40 as the emitter and can 14 asthe collector, and using RV coil 36 and can 14 as voltage measuringpoints.

Impedance changes may also be measured across a vector, Vector 3, formedby a stimulation path and a sense path from RV ring electrode 40 to can14. Vector 3 is a ventricular bi-polar vector, utilizing rightventricular ring electrode 40 as the emitter and can 14 as thecollector, and also using right ventricular ring electrode 40 and can 14as voltage measuring electrodes. Impedance changes may also be measuredacross another vector, Vector 4, formed by a stimulation path from RVring electrode 40 to can 14 and a sense path from RV tip electrode (Vt)44 to can 14. Vector 4 is a ventricular tri-polar vector, utilizingright ventricular ring electrode 40 as the emitter and can 14 as thecollector, and also using right ventricular tip electrode 44 and can 14as voltage measuring electrodes. Another vector may be used, Vector 5,formed by a stimulation path from RV coil electrode 40 to can 14 and asense path from RV coil electrode 40 to button 158. Vector 5 is atri-polar vector, using right ventricular coil 40 as the emitter and can14 as the collector, and utilizing right ventricular coil 40 and button158 on the can 14 as voltage measuring electrodes.

The above described vectors are but a few of the possible leads,electrodes, and vectors that can be used according to the presentinvention. Other electrodes that can be used include superior vena cavacoils, right atrial ring electrodes, right atrial tip electrodes, leftatrial coils, left atrial ring electrodes, left atrial tip electrodes,left ventricular ring electrodes, left ventricular tip electrodes,including leads place via the coronary sinus, along with electrodesplaced endocardially or epicardially. Several impedance measuringelectrodes and vectors that can be used to advantage in the presentinvention are discussed in U.S. Published Patent Application No.2002/0002389, herein incorporated by reference. More combinations can bevisually created by inspection and are well known to the inventor andwill become apparent to those skilled in the art. Additionalcombinations can be created using additional electrodes not limited tothose shown in the Figures.

Vector 6 is an AV quadra-polar vector, utilizing right ventricular ringelectrode 40 as an emitter and right atrial ring electrode 32 as acollector, and using a right ventricular tip 44 and right atrial tip 28as the voltage measuring electrodes. Vector 7 is a brady, tri-polarvector utilizing right atrial ring electrode 32 as the emitter and thecan 14 as a collector, and utilizing right atrial tip 28 and can 14 asvoltage measuring electrodes.

FIG. 4 illustrates a table or susceptibility matrix for the variousvectors previously described, along with two others, not requiringseparate illustration and well known to those skilled in the art. FIG. 4includes the sensitivities or susceptibilities of the various vectors tothe various physiological impedance factors, as will be discussedfurther. The various factors included in FIG. 4 are lung resistivity,blood resistivity, heart muscle resistivity, skeletal muscleresistivity, heart volume, and lung volume. Vectors 1 through 7 are aspreviously described. The column labeled “Vector” in FIG. 4 includes thestimulation electrode pair/sense electrode pair. Vector 2 thus refers tostimulation between the right ventricular ring and can, and sensingbetween the right ventricular coil and can. Inspection of FIG. 4 shows,for example, that Vector 3 is extremely sensitive to changes in bloodresistivity relative to the other various physiological factors. Vector2 may be seen to be much less sensitive to changes in blood resistivitythan Vector 3. It may also be seen that Vectors 1 and 2 varysignificantly in the sensitivity to blood resistivity, while having verysimilar sensitivities to the remaining factors. The sensitivities orsusceptibilities for Vectors 1 through 4 and 6 through 7 have beentheoretically derived from mathematical modeling, and validated. Thesensitivities or susceptibilities can be further refined and calibratedthrough testing by those skilled in the art, using the teachings of thepresent invention. Vector 5 contains values in FIG. 4 that have beenestimated based on physical physiological considerations and the othervalues in the table.

Equation 13 below gives the change in impedance over time for a selectedvector as a function of the sensitivities or susceptibilities of afactor in FIG. 4. There may be other factors for which there aresubstantial impedance contributions but for which there are nosubstantial impedance change contributions. One such example is thedistance between two electrodes for which the distance is expected toremain fixed. As used in the present application, the impedancecontributions refer to impedance contributions for which changes can beexpected over time.ΔZ=α_(L) *Q _(L)+α_(B) *Q _(B)+α_(HM) *Q _(HM)+α_(SM) *Q _(SM)+α_(HV) *K_(HV)+α_(LV) *K _(LV)  (13)

Q is equal to Δρ/ρ, and K is equal to ΔV/V. L represents lungresistivity, B represents blood resistivity, HM represents heart muscleresistivity, SM represents skeletal muscle resistivity, HV representsheart volume, and LV represents lung volume. As will be discussed below,in some methods, the lung resistivity and heart volume resistivity, Land HV may be lumped together as a single parameter as an indicator ofheart failure, as is the case with fluid overload in congestive heartfailure.

Using the values of the table in FIG. 4 together with equation 13, andthe various methods previously described for the general statement ofthe invention, we may now derive physiologically meaningful changes infactors.

The changes in blood resistivity are often of interest to a treatingphysician. Changes in blood resistivity can indicate electrolyteimbalances and also the effectiveness of blood thinners or otherprescribed medications. Inspection of FIG. 4 shows that Vectors 1 and 2differ in the sensitivity to changes in blood resistivity but havesubstantially the same sensitivities as between the two vectors to theother factors in FIG. 4. This indicates that Vectors 1 and 2 may beevaluated to solve for the fractional change in blood resistivity.Inserting the values for Vector 1 into equation 13 and the values forVector 2 into equation 13 allows us to solve for Q_(B). The impedancecan be measured across Vector 1 at time 1 and the impedance measuredacross Vector 2 also at time 1, or a very short time after time 1, forexample, microseconds after time 1. At a later time, for example, hours,days or weeks later, the impedance across Vectors 1 and 2 may beevaluated at time 2. The change in impedance over time for Vector 2 maybe subtracted from the change in impedance over time for Vector 1,leading us to the result of equation 14 below.ΔZ _(V1) −ΔZ _(V2)=0*Q _(L)+(0.13−0.023)*Q _(B)−0.01*Q _(HM)+0*Q_(SM)−0K _(HV)+0.002*K _(LV)  (14)

The contribution difference by Q_(HM) is small and may be neglected, asmay be the contribution difference by K_(LV). Solving for Q_(B),Q_(B)=(ΔZ_(V1)−ΔZ_(V2))/0.107. Q_(B) has thus been determined usingequation 13, the susceptibility matrix table, and the measurements fromVectors 1 and 2. The mathematics involved in determining Q_(B) can beimplemented in several ways. In some methods, the impedance changes overtime are periodically measured by the implanted medical device andstored. The stored values can be retrieved periodically or on demand bya telemetry device. The telemetry device itself, or a separate computingdevice, or the implanted device itself, can implement theabove-described methods in order to determine the change in bloodresistivity over time. This change in blood resistivity, or any otherfactor according to the present invention, may be plotted, analyzed, andtransmitted to a treating physician for further analysis. A significantchange in the blood resistivity, or any other factor in the presentinvention, may be flagged or indicated as deserving particularattention. Some methods alert the patient and/or a treating physicianvia a patient alert system, which can include a computer network,including the Internet and Websites, in either or both directionsbetween patient and physician.

The relative change in heart muscle resistivity is also of interest. Theresistivity of the heart muscle can change as a function of the degreeof perfusion of the heart muscle. A decrease in perfusion, for example,caused by a decrease in blood being supplied by the coronary arteries,can be indicative of significant blockage or of myocardial infarction.The change in the heart muscle relative resistivity is thus a factor ofparticular interest. Inspection of FIG. 4 shows that Vectors 2 and 3differ in their sensitivity to changes in heart muscle resistivity,while remaining approximately the same for other substantiallycontributing factors. Vectors 2 and 3 do differ in their sensitivity toblood resistivity, but the change in blood resistivity, Q_(B), haspreviously been solved. The values from FIG. 4 for Vector 2 and Vector 3may be substituted into Equation 13. Equation 13 evaluated at Vector 2may then be subtracted from the values for Equation 13 for Vector 3,resulting in Equation 15.ΔZ _(V3) −ΔZ _(V2)=0.020*Q _(L)+1.257*Q _(B)+0.44*Q _(HM)+0.04*Q_(SM)+0.0003*K _(HV)+0.003*K _(LV)  (15)

The value for Q_(B) is already known. The contributions for Q_(L),Q_(SM), K_(HV), and K_(LV) are significantly less than those of Q_(B)and Q_(HM), and may therefore be initially treated as 0. Using thepreviously obtained value for Q_(B), Equation 16 results, solving forQ_(HM).Q _(HM)=((ΔZ _(V3) −ΔZ _(V2))−1.257*Q _(B))/0.44  (16)

Q_(HM) has thus been solved for, providing an indication of heart muscleperfusion. As discussed with respect to other factors, the relative orfractional changes in Q_(HM) can be determined by measuring the changesin impedance over time across Vectors 2 and 3, with the changes inQ_(HM) automatically computed and analyzed.

The changes in skeletal muscle resistivity, Q_(SM), are also ofinterest. A significant change in the skeletal muscle resistivity can beindicative of inflammation or edema of muscle surrounding the pocketcontaining the implanted medical device. A change in Q_(SM) can beindicative by hematoma, bleeding in the pocket. A significant change inQ_(SM) can also be indicative of infection in the pocket.

Inspection of FIG. 4 shows that Vector 5 has a significant difference insensitivity for skeletal muscle relative to the other vectors. Vector 5,as previously discussed, is an estimate of the expected values for thesensitivities. It may be noted that the values for the blood resistivityand heart muscle resistivity may not be of importance as to their exactvalues as the values for Q_(B) and Q_(HM) are already known. What issignificant is that the changes in sensitivity for skeletal muscle ofVector 5 relative to the other vectors is a significantly largedifference. Using the methods previously described, Q_(SM) may be solvedfor by evaluating Equation 13 for Vector 5 and another vector, forexample, Vector 1. When the differences in Equation 13 for Vectors 1 and5 are evaluated, with the values for Q_(B) and Q_(HM) already beingknown, and the sensitivity differences in lung resistivity, heartvolume, and lung volume being extremely small, Q_(SM) can be solved for.Given the values in FIG. 4, another method solves for Q_(SM) usingVectors 1 and 5 without requiring knowledge of any other factors.Evaluating the change in Q_(SM) thus provides an indication of hematomaor infection in the pocket, which can be indicated as a change ofinterest to the treating physician.

The value for K_(LV), the fractional change in lung volume, may also beevaluated using equations according to the present invention and theproper sensitivity coefficients. Inspection of FIG. 4 indicates a smallchange in sensitivities or a difference in sensitivities between Vectors1 and 2. These differences in sensitivities are small, relative to thedifferences previously encountered for the other factors. This smalldifference in sensitivities means that the resulting value may beeffected by noise and uncertainty in the values. The accuracy of theresulting K_(LV) value will thus likely be less accurate using only thesensitivity values found in FIG. 4. Nonetheless, K_(LV) can be solvedfor by substituting the values for Vector 1 and Vector 2 into Equation13 and subtracting the values for Vector 1 from the values for Vector 2.The values for the sensitivity of heart volume are equal to each otheras between Vectors 1 and 2, thus removing heart volume as a factor inthe equation. The resulting K_(LV) can give an indication in changesover time for the average lung volume. Q_(B), Q_(HM) and Q_(SM) havebeen previously solved, and K_(LV) can be determined, and tracked, withthe changes noted and reported over time.

The changes in lung resistivity and heart volume, Q_(L) and K_(LV), areof interest as a group, as they are indicative of heart failure. WithEquation 13 thus solved for all factors but lung resistivity and heartvolume change, a change in impedance over time may thus have the bloodresistivity, heart muscle resistivity, skeletal muscle resistivity, andlung volume change accounted for, leaving only the lung resistivity andheart volume change on one side of the equation. The combined lungresistivity and heart volume change may thus be tracked as well, as agroup. The changes in the combined lung resistivity and heart volumechange may also be tracked over time, with significant changes noted,reported, and further analyzed by a treating physician. This combinedchange can be of particular value in tracking congestive heart failure.

FIG. 5 is a flowchart illustrating a method for isolating impedancechanges over time to monitor physiological factors according to thepresent invention. According to the present invention, an implantablemedical device utilizing the method for identifying cardiac insult ofthe present invention can be programmed to determined changes in all orany number of the factors listed in the table of FIG. 4. For example, asillustrated in FIG. 5, a method for monitoring a plurality ofphysiological factors contributing to physiological conditions of apatient, according to the present invention includes measuring impedancealong any number of the vectors in the table of FIG. 4, Step 200,waiting a predetermined time period, such as hours, days, weeks, Step202, and measuring the impedance along the vectors again, Step 204.Based on the two measured impedances along the predetermined vectors, arelative change in impedance is determined, Step 206. Using the table ofFIG. 4, the desired programmed physiological factors of thephysiological factors included, such as lung resistivity, bloodresistivity, heart muscle resistivity, skeletal muscle resistivity,heart volume and lung volume, are identified, and minimally contributingfactors are determined for the programmed physiological factors, Step208. Relative change in resistivity for the programmed physiologicalfactors is then determined, Step 210, and the results are stored, oroutput to an external device, such as a programmer, a network, a datatransmission bus, or a patient alert device, Step 212.

For example, if the desired programmed physiological factor is bloodresistivity, impedance is measured along vectors 1 and 2 of the Table inFIG. 4, and the minimally contributing factors are determined to be lungresistivity, heart muscle resistivity, skeletal muscle resistivity,heart volume and lung volume. The relative change in resistivity forthis physiological factor is then determined using the equation forobtaining blood resistivity Q_(B)=(ΔZ_(V1)−ΔZ_(V2))/0.107 obtained fromequation 13 as described above, with ΔZ_(V1)−ΔZ_(V2) being equal to therelative change determined in Step 206.

If the desired physiological factor is heart muscle resistivity,impedance is measured along vectors 1, 2 and 3 of the Table in FIG. 4,and the minimally contributing factors are determined to be lungresistivity, skeletal muscle resistivity, heart volume and lung volume.The relative change in resistivity is determined for blood resistivity,and the relative change in resistivity is determined for heart muscleresistivity using equation (16) as described above, with ΔZ_(V3)−ΔZ_(V2)being equal to the relative change in impedance determined in Step 206.

In the same way, if the desired physiological factor is skeletal muscleresistivity, impedance is measured along vectors 1, 2, 3 and 5 of theTable in FIG. 4, and the minimally contributing factors are determinedto be lung resistivity, heart volume and lung volume. The relativechange in resistivity is determined for skeletal muscle using valuesdetermined for blood resistivity and heart muscle resistivity, usingEquation 13, with the relative change in impedance determined in Step206 being ΔZ_(V5)−ΔZ_(V1) if vectors 1 and 5 are utilized,ΔZ_(V5)−ΔZ_(V2) if vectors 2 and 5 are utilized (and Q_(B) is determinedusing vectors 1 and 2 as described above), ΔZ_(V5)−ΔZ_(V3) if vectors 3and 5 are utilized (and Q_(B) and Q_(HM) are determined using vectors1–3), ΔZ_(V5)−ΔZ_(V4) if vectors 4 and 5 are utilized (and Q_(B) andQ_(HM) are determined using vectors 1–3). As described below, lungresistivity and heart volume are computed in the same way using Equation13, with vectors 6 and 7 being utilized so that the relative change inimpedance determined in Step 206 is ΔZ_(V6)−ΔZ_(V5) for example, and thevalues for the remaining factors previously obtained are used.

The present invention may be extended by those skilled in the art frominspection of the location of various leads. In one example, a vectorfrom a first button on the can to a second button on the can is unlikelyto be sensitive to changes in lung volume. Similarly situated electrodesare likely to have similar sensitivities to the same factor, even whenthe sensitivities are substantial. In another example, a vector from theRV coil and SVC coil will be more sensitive to heart volume, and muchless sensitive to skeletal muscle changes.

The present invention explicitly includes within its scope implantablecardiac devices executing programs or logic implementing methodsaccording to the present invention. The present invention's scope alsoincludes computer programs or logic capable of being executed, directlyor indirectly, on implantable medical device impedance data. Computerreadable media having instructions for implementing or executing methodsaccording to the present invention are also within the scope of thepresent invention. Impedance factor isolating methods, devicesimplementing those methods, computer programs implementing thosemethods, and computer readable media containing programs implementingthose methods are also within the scope of the invention. The computerreadable medium includes any type of computer readable memory, such asfloppy disks, conventional hard disks, CD-ROMS, Flash ROMS, nonvolatileROM, and RAM.

While a particular embodiment of the present invention has been shownand described, modifications may be made. It is therefore intended inthe appended claims to cover all such changes and modifications, whichfall within the true spirit and scope of the invention.

1. A patient monitoring system comprising: an implantable medical device comprising: a housing and a connector block configured to couple to a cardiac lead system having a plurality of electrodes; means for selecting electrodes of a cardiac lead system to establish an impedance vector in tissue proximate a patient's heart; means coupled to the electrodes selecting means for measuring impedance of tissue proximate a patient's heart based on an impedance vector formed between electrodes of a cardiac lead system; and means for determining a quantifying value for a contributing physiological impedance factor among a plurality of physiological impedance factors associated with a physiological condition of a patient's heart, wherein the means for determining a quantifying value for a contributing physiological impedance factor among a plurality of physiological impedance factors associated with a first physiological condition of a patient's heart among a plurality of physiological conditions of a patient's heart comprises: a microprocessor operating to (1) cause the means for measuring impedance of tissue proximate a patient's heart based upon an impedance vector formed between electrodes of a cardiac lead system to make first and second impedance measurements spaced apart in time along a first impedance vector and to make first and second impedance measurements spaced apart in time along a second impedance vector; (2) calculate a value for a change in measured tissue impedance over time along each of the first and second impedance vectors as ΔZ_(V1) and ΔZ_(V2), respectively; (3) insert each of the calculated values ΔZ_(V1) and ΔZ_(V2) into an equation ΔZ=α _(L) *Q _(L)+α_(B) *Q _(B)α_(HM) *Q _(HM)+α_(SM) *Q _(SM)+α_(HV) *K _(HV)+α_(LV) *K _(LV), where Q_(L) is lung tissue fractional resistivity change, Q_(B) is blood fractional resistivity change, Q_(HM) is heart muscle fractional resistivity change, Q_(SM) is skeletal muscle fractional resistivity change, K_(HV) is heart volume fractional change, K_(LV) is lung volume fractional change, and each of Q_(L), Q_(B), Q_(HM), Q_(SM), K_(HV), and K_(LV) is a physiological impedance factor, α_(L) is lung tissue impedance sensitivity factor, α_(B) is blood impedance sensitivity factor, α_(HM) is heart muscle impedance sensitivity factor, α_(SM) is skeletal muscle impedance sensitivity factor, α_(HV) is heart volume impedance sensitivity factor, α_(LV) is lung volume impedance sensitivity factor; (4) subtract ΔZ_(V2) from ΔZ_(V1) to form the equation ΔZ _(V1) −ΔZ _(V2)=(α_(LV1)−α_(LV2))*Q _(L)+(α_(BV1)−α_(BV2))*Q _(B)+(α_(HMV1)−α_(HMV2))*Q _(HM)+(α_(SMV1)−α_(SMV2))*Q _(SM)+(α_(HVV1)−α_(HVV2))*K _(HV)+(α_(LVV1)−α_(LVV2))*K _(LV); and (5) solve for one of the physiological impedance factors Q_(L), Q_(B), Q_(HM), Q_(SM), K_(HV), and K_(LV) using the equation.
 2. The system of claim 1, wherein the plurality of physiological impedance factors includes lung resistivity, blood resistivity, heart muscle resistivity, skeletal muscle resistivity, heart volume and lung volume, and wherein the contributing physiological impedance factor is blood resistivity.
 3. The system of claim 1, wherein the plurality of impedance vectors includes a first vector and a second vector, the first vector including a first stimulation path and a first sense path extending between a first electrode of the plurality of electrodes, positioned within a ventricle of the heart, and an uninsulated portion of the housing, and the second vector including a second stimulation path, extending between a second electrode of the plurality of electrodes, positioned within a ventricle of the heart, and the uninsulated portion of the housing, and a second sense path extending between the first electrode and the uninsulated portion of the housing.
 4. The system of claim 1, wherein the plurality of impedance vectors include a first vector and a second vector, the first vector including a first stimulation path and a first sense path extending between a first electrode of the plurality of electrodes, positioned within a ventricle of the heart, and an uninsulated portion of a housing of the device, and the second vector including a second stimulation path extending between the first electrode and the uninsulated portion of the housing and a second sense path extending between the first electrode and a second electrode of the plurality of electrodes, positioned along the housing.
 5. The system of claim 4, wherein the plurality of physiological impedance factors include lung resistivity, blood resistivity, heart muscle resistivity, skeletal muscle resistivity, heart volume and lung volume, and wherein the contributing physiological impedance factor is skeletal muscle resistivity.
 6. The system of claim 1, wherein the contributing physiological impedance factor is one of a group consisting of lung resistivity, blood resistivity, heart muscle resistivity, skeletal muscle resistivity, heart volume, and lung volume.
 7. The system of claim 1 wherein the means for selecting electrodes of a cardiac lead system to establish an impedance vector in tissue proximate a patient's heart comprises an electrode switch matrix and a controller.
 8. The system of claim 1 further comprising means for establishing a pattern of sensitivities of physiological impedance factors that contribute to tissue impedance, the pattern being established according to each of the plurality of impedance vectors formed between a plurality of electrodes of a cardiac lead system.
 9. The system of claim 1 wherein the means for establishing a pattern of sensitivities of physiological impedance factors that contribute to tissue impedance comprises a look-up table of coefficient values.
 10. The system of claim 7 wherein the means for measuring impedance in tissue proximate a patient's heart comprises an impedance measurement interface coupled to the electrode switch matrix.
 11. The system of claim 1 wherein the plurality of electrodes of the cardiac lead system comprises a right ventricular (RV) coil, a right ventricular (RV) ring, right ventricular (RV) tip, an uninsulated portion of the housing, and a button electrode positioned along the housing; and wherein the means for selecting electrodes of a cardiac lead system to establish an impedance vector in tissue proximate a patient's heart forms an impedance vector from a group of impedance vectors consisting of (1) a stimulation path and a sense path between the RV coil and the housing, (2) a stimulation path from the RV ring to the housing and a sense path from the RV coil to the housing, (3) a stimulation path and a sense path from the RV ring to the housing, (4) a stimulation path from the RV ring to the housing and a sense path from the RV tip to the housing, (5) a stimulation path from the RV coil to the housing and a sense path from the RV coil to the button electrode.
 12. The system of claim 1 further comprising: a memory coupled to the means for determining a quantifying value for a contributing physiological impedance factor, the memory accepting quantifying values for storage and subsequent retrieval.
 13. A patient monitoring system comprising: an implantable medical device comprising: a housing and a connector block configured to couple to a cardiac lead system having a plurality of electrodes; means for selecting electrodes of a cardiac lead system to establish an impedance vector in tissue proximate a patient's heart; means coupled to the electrodes selecting means for measuring impedance of tissue proximate a patient's heart based on an impedance vector formed between electrodes of a cardiac lead system; and means for determining a quantifying value for a contributing physiological impedance factor among a plurality of physiological impedance factors associated with a physiological condition of a patient's heart, wherein the means for determining a quantifying value for a contributing physiological impedance factor among a plurality of physiological impedance factors associated with a first physiological condition of a patient's heart among a plurality of physiological conditions of a patient's heart comprises: a processor operating to (1) cause the means for measuring impedance of tissue proximate a patient's heart based upon an impedance vector formed between electrodes of a cardiac lead system to make first and second impedance measurements spaced apart in time along a first impedance vector and to make first and second impedance measurements spaced apart in time along a second impedance vector; (2) calculate a value for a change in measured tissue impedance over time along each of the first and second impedance vectors as ΔZ_(V1) and ΔZ_(V2), respectively; (3) insert each of the calculated values ΔZ_(V1) and ΔZ_(V2) into an equation ΔZ _(VX)=α_(AVX) *Q _(A)+α_(BVX) *Q _(B) where Q_(A) is a first fractional resistivity change of a physiological impedance factor, Q_(B) is a second fractional resistivity change of a physiological impedance factor, α_(AVX) is an impedance sensitivity factor for physiological impedance factor Q_(A), and α_(BVX) is an impedance sensitivity factor for physiological impedance factor Q_(B); (4) subtract ΔZ_(V2) from ΔZ_(V1) to form the equation ΔZ _(V1) −ΔZ _(V2)=(α_(AV1)−α_(AV2))*Q_(A)+(α_(BV1)−α_(BV2))*Q _(B); and (5) solve for a quantifying value for one of the physiological impedance factors Q_(A) and Q_(B) using the equation.
 14. The system of claim 13 wherein Q_(A) and Q_(B) are selected from a group of physiological impedance factors consisting of lung resistivity, blood resistivity, heart muscle resistivity, skeletal muscle resistivity, heart volume and lung volume.
 15. The system of claim 13 wherein Q_(A) and Q_(B) are fractional changes in resistivity and given by the equations Q_(A)=Δρ_(A)/ρ_(A)=(ρ_(AT2)−ρ_(AT1))/ρ_(AT1) and Q_(B)=Δρ_(B)/ρ_(B)=(ρ_(BT2)−ρ_(BT1))/ρ_(BT1).
 16. The system of claim 13 wherein the processor is a microprocessor executing a set of instructions stored in a memory.
 17. The system of claim 16 further comprising a computer-readable medium having the set of executable instructions for downloading into the memory.
 18. The system of claim 13 further comprising a memory coupled to the processor to accept a physiological impedance factor quantifying value for storage and subsequent retrieval. 